Methods and Pressure Vessels for Solid-State Microcellular Processing of Thermoplastic Rolls or Sheets

ABSTRACT

Disclosed herein are methods and pressure vessels for solid-state microcellular processing of thermoplastic rolls and sheets. In one embodiment, the present invention is directed to a method for making a gas impregnated interleaved roll, which method comprises: providing a pressure vessel having an internal pressure chamber and a rotatable shaft horizontally positioned within the pressure chamber; placing an interleaved roll about the rotatable shaft and within the pressure chamber, wherein the interleaved roll is made from a thermoplastic material sheet interleaved together with a gas-channeling material sheet; pressurizing the pressure chamber to a selected pressure; rotating the rotatable shaft having the interleaved roll thereabouts (thereby rotating the interleaved roll) while under pressure for a selected period of time; and depressurizing the internal chamber to yield the gas impregnated interleaved roll. In other embodiments, the invention is directed to multi-chambered pressure vessels for gas impregnation of thermoplastic rolls, sheets, and films.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/061,539 filed on Jun. 13, 2008, which application is incorporatedherein by reference in its entirety for all purposes.

TECHNICAL FIELD

The present invention relates generally to the foaming of plasticmaterials and, more specifically, to methods and pressure vessels forsolid-state microcellular processing of thermoplastic rolls, sheets orfilms.

BACKGROUND OF THE INVENTION

Microcellular plastic-foam refers to a polymer that has been speciallyfoamed to thereby create micro-pores or cells (also sometime referred toas bubbles). The common definition includes foams having an average cellsize on the order of 10 microns in diameter, and typically ranging fromabout 0.1 to about 100 microns in diameter. In comparison, conventionalplastic foams typically have an average cell diameter ranging from about100 to 500 microns. Because the cells of microcellular plastic foams areso small, to the casual observer these specialty foams generally retainthe appearance of a solid plastic.

Microcellular plastic foams can be used in many applications such as,for example, insulation, packaging, structures, and filters (D. Klempnerand K. C. Fritsch, eds., Handbook of Polymeric Foams and FoamTechnology, Hanser Publishers, Munich (1991)). Microcellular plasticfoams have many unique characteristics. Specifically, they offersuperior mechanical properties at reduced material weights and costs.

The process of making microcellular plastic foams has been developedbased on a thermodynamic instability causing cell nucleation (J. E.Martini, S M Thesis, Department of Mech. Eng., MIT, Cambridge, Mass.(1981)). First, a polymer is saturated with a volatile foaming agent ata high pressure. Then, by means of a rapid pressure drop, the solubilityof foaming agent impregnated within the polymer is decreased, and thepolymer becomes supersaturated. The system is heated to soften thepolymer matrix and a large number of cells are nucleated. The foamingagent diffuses outwards and into a large number of small cells. Statedsomewhat differently, microcellular plastic foam may be produced bysaturating a polymer with a gas or supercritical fluid and using athermodynamic instability, typically a rapid pressure drop, to generatebillions of cells per cubic centimeter (i.e., bubble density of greaterthan 10⁸ cells per cubic centimeter) within the polymer matrix.

Conventional solid-state microcellular processing is known to involve atwo-stage batch process. In the first stage (absorption), a solidpolymer is saturated with high pressure inert gas (e.g., CO₂) in apressure vessel until a desired gas concentration level is achievedthroughout the polymer matrix. Once the gas-polymer mixture is removedfrom the pressure vessel into ambient environment (desorption), asupersaturated specimen is produced that is thermodynamically unstabledue to the excessive concentration of gas in the polymer. In the secondstage (foaming), the gas-polymer mixture is heated in a hot water bathor some other heating medium (e.g., hot air, steam, infrared radiation,etc.) at a temperature close to the glass transition temperature (T_(g))of the gas-polymer mixture in order to induce microcellular bubblenucleation and growth.

The success of the batch process in producing discrete units ofthermoplastic material has not, however, been duplicated in large scaleproduction involving continuous rolls, sheets or films of thermoplasticmaterial. To scale-up the batch process for industrial production,several patented methods have been issued for thermoplastic processing.Exemplary in this regard are the following:

U.S. Pat. No. 5,158,986 to Cha et al. (issued Oct. 27, 1992) disclosesthe formation of microcellular plastic foams by using a supercriticalfluid as a blowing agent. In a batch process, a plastic article issubmerged at pressure in a supercritical fluid for a period of time, andthen quickly returned to ambient conditions so as to create a solubilitychange and nucleation. In a continuous process, a polymeric sheet isextruded and run through a system of time-controlled rollers within acontainer of supercritical fluid at pressure, and then exposed quicklyto ambient conditions. Dynamic seals are stationed between the chambersto allow passage of the thermoplastic sheet while preserving theenvironmental conditions of each chamber.

The breakthrough in large scale solid-state microcellular thermoplasticproduction is disclosed in U.S. Pat. No. 5,684,055 to Kumar et al.(issued Nov. 4, 1997), which patent discloses a method for thesemi-continuous production of microcellular foamed articles. Asdisclosed, a roll of polymer sheet is provided with a gas channelingmeans (e.g., gauze, paper towel) interleaved between the layers ofpolymer. The interleaved roll is exposed to a non-reacting gas atelevated pressure for a period of time sufficient to achieve a desiredconcentration of gas within the polymer. The saturated polymer sheet isthen separated from the gas channeling means and bubble nucleation andgrowth is initiated by heating the polymer sheet (FIG. 1). A limitationof the semi-continuous method, as acknowledged by Kumar, is that only afinite length of solid thermoplastic material may be processed at onetime (to ensure that it is foamed promptly before too much gas escapesthe material during its time under ambient conditions, a factor thatcould lead to undesirable variations in foam density).

U.S. Patent Application Publication No. US2005/0203198 to Branch et al(published. Sep. 5, 2005) discloses another semi-continuous solid-stateprocess that utilizes gas impregnation (similar to that of Kumar et al.)under specialized conditions to enhance foaming and thermoformmg of thethermoplastic material.

While the semi-continuous methods as taught by Kumar and Branch addressone factor associated with uneven gas concentrations of a thermoplasticroll (namely, within-roll variation in absorption rates of exposed vs.non-exposed surfaces to high pressure gas), there are still otherfactors that can cause unwanted variations in gas concentration duringthe absorption or desorption phase or both (which may result in unevenlyfoamed thermoplastic products). For example, within-roll variation ingas concentration during absorption is believed to be, in part, afunction of the stress and volume dilation of the gas impregnated roll(and is applicable to all polymer types).

In addition, high pressure gas saturation of an interleavedthermoplastic roll may also cause: (1) the thermoplastic roll to becomeheavier and softer (hence weaker), thereby resulting in sagging andgreater stress at the top portion of the roll (FIG. 2) and even furtherwithin-roll variation in gas absorption rates and gas concentrationlevels between the top (A) and bottom (B) portions of the roll; (2)volume dilation of the roll whereby the roll expands in volume andbegins to compress the interleaved medium to the extent that it losesporosity and, consequently, its gas-permeation function.

Moreover, a saturated interleaved thermoplastic roll with fast gaseousdiffusion, whether due to the class of polymer, like polylactic acid(PLA) and polystyrene (PS), or due to the thin dimension (<0.010 inch)of a polymer with moderate diffusivity, tends to desorb gas quickly onceit is moved into ambient environment and must be heated substantiallyimmediately to obtain even foaming. If the time between absorption andfoaming exceeds the narrow window of processability, within-rollvariation of gas concentration across the length of the roll, relativeto which end is heated first, may lead to uneven bobble growth and sizeresulting in a non-uniform microcellular foamed structure andnon-uniform density.

Saturated interleaved thermoplastic rolls with moderate gas diffusivity,like polyethylene terephthalate (PET) and polycarbonate (PC), areallowed a longer time between absorption and foaming because they desorbgas at slower rates under ambient conditions. However, between-rollvariation, in gas concentration may nevertheless occur if a batch ofthermoplastic rolls of moderate gaseous diffusion, which have beensaturated with gas and removed from the pressure vessel at the sametime, sit too long under ambient conditions on queue to be heated. Eachthermoplastic roll that is subsequently foamed has been exposed toambient conditions longer and thus may experience Incrementally higherredactions in gas concentration. Significant variation in gasconcentration between successively heated rolls my result in uneven foamquality among the batch of foamed thermoplastic rolls. The problem ofuneven gas concentration in the continuous production of solid-statemicrocellular thermoplastics is due, in part, to limitations of existingmethods and apparatuses (meaning that such methods and apparatuses arenot designed to control for any untoward physical events in thethermoplastic material during absorption nor to respond to downstreamprocessing flow by regulating the desorption time). For instance,current pressure vessels used to saturate thermoplastics with highpressure gas are typically designed as a single-chamber cavity with asingle-door opening to allow for the insertion and removal of thetreated sample. In one embodiment of a single-door, single-chamberpressure vessel, a plurality of interleaved thermoplastic rolls arehoused inside the pressure chamber where one or more inlet valves injecthigh pressure gas (e.g., CO₂) into the chamber saturating thethermoplastic rolls until they obtain the desired gas concentrationlevel. Outlet valves then evacuate the gas from the chamber and the dooris swung open to remove the saturated rolls from the pressure vessel.This single-door, single-chamber pressure vessel, which constrains theabsorption process to a uniform time frame where interleavedthermoplastic rolls are placed inside the pressure vessel at T₁,saturated with gas at T₂, and evacuated at T₃, is not designed fortime-sensitive, downstream processing flow.

In another embodiment of the single-door, single-chamber pressurevessel, a plurality of interleaved thermoplastic rolls that have beensaturated inside the pressure chamber are taken out of the pressurevessel at different times. Before any of the interleaved thermoplasticrolls can be removed, the pressure vessel must first be depressurizedand evacuated of gas. This change inside the pressure vessel environmentmeans that the processing condition of the remainder rolls has beensignificantly interrupted by depressurization, gas desorption, andsubsequent repressurization. In this instance, the single-door,single-chamber pressure vessel is unequal to the demands of productionflow while maintaining a constant pressurized environment.

Accordingly, there is a need in the art for novel methods andapparatuses for continuous production of foamed thermoplastic material,with consistent quality in microcellular structure and foam density. Thepresent invention fulfills these needs and provides for further relatedadvantages.

SUMMARY OF THE INVENTION

In brief, the present invention relates to methods and pressure vesselsfor solid-state microcellular processing of thermoplastic rolls, sheets,and films. In one embodiment, a method for making a gas impregnatedinterleaved roll comprises: providing a pressure vessel having aninternal pressure chamber and a rotatable shaft horizontally positionedwithin the pressure chamber; placing an interleaved roll about therotatable shaft and within the pressure chamber, wherein the interleavedroll is made from a thermoplastic material sheet interleaved togetherwith a gas-channeling material sheet; pressurizing the pressure chamberto a selected pressure; rotating the rotatable shaft having theinterleaved roll thereabouts while under pressure for a selected periodof time; and depressurizing the internal chamber to yield the gasimpregnated interleaved roll.

In another embodiment, a method for making a gas impregnated interleavedroll comprises: providing a pressure vessel having an internalpass-through multi-chambered cavity, wherein the multi-chambered cavityincludes at least two outer pressure chambers sandwiching a centralpressure chamber, and wherein a rotatable shaft is horizontallypositioned within each of the two outer and central pressure chambers ofthe multi-chambered cavity; placing an interleaved roll about therotatable shaft and within one of the two outer pressure chambers,wherein the interleaved roll is made from a thermoplastic material sheetinterleaved together with a gas-channeling material sheet; pressurizingthe one of the two outer pressure chamber to a first selected pressure;rotating the rotatable shaft having the interleaved roll thereaboutswhile under the first selected pressure for a first selected period oftime; conveying the interleaved roll to the central pressure chamberwhile under pressure; rotating the rotatable shall having theinterleaved roll thereabouts while under a second selected pressure fora second selected period of time; and depressurizing the other one ofthe two outer pressure chambers to yield the gas impregnated interleavedroll.

In yet another embodiment, a pressure vessel having an internal,pass-through multi-chambered cavity for gas impregnation of interleavedrolls comprises: at least two outer pressure chambers sandwiching acentral pressure chamber, wherein the central pressure chamber isseparated from each of the two outer pressure chambers by respectiveinterior doors, and wherein each of the two outer and central pressurechambers are separably procurable to selected first, second, and thirdpressures, respectively; and a rotatable shaft horizontally positionedwithin each of the two outer and central pressure chambers and througheach of the interior doors of the multi-chambered cavity.

In still yet another embodiment, a pressure vessel having an internalpass-through multi-chambered cavity for gas impregnation of anintermittently fed thermoplastic material sheet comprises: at least twoouter pressure chambers sandwiching a central pressure chamber, whereinthe central pressure chamber is separated from each of the two outerpressure chambers by respective interior sealable pass-through slots,wherein each of the interior sealable pass-through slots is defined by arectangular opening having a pair of adjacently positioned confrontingstatic seals for sealably engaging the intermittently fed thermoplasticmaterial sheet, and wherein each of the two outer and central pressurechambers are separably pressurable to selected first, second, and thirdpressures, respectively; a sealable inlet at a first end portion of thepressure vessel, wherein the sealable inlet is defined by a rectangularinlet slot having a pair of adjacently positioned confronting staticseals for sealably engaging the intermittently fed thermoplasticmaterial sheet; a sealable outlet at a second end portion of thepressure vessel, wherein the sealable inlet is defined by a rectangularoutlet slot having a pair of adjacently positioned confronting staticseals for sealably engaging the intermittently fed thermoplasticmaterial sheet; and a roller system (which may reside partially withinand partially outside multi-chambered cavity) for intermittentlyconveying the thermoplastic material sheet through the sealable inlet,the outer and central pressure chambers by way of the interior sealablepass-through slots, and the sealable outlet.

In still yet another embodiment, a method for gas impregnation of anintermittently fed thermoplastic material sheet comprises: providing apressure vessel having at least two outer pressure chambers sandwichinga central pressure chamber, wherein the central pressure chamber isseparated from each of the two outer pressure chambers by respectiveinterior sealable pass-through slots, wherein each of the interiorsealable pass-through slots is defined by a rectangular opening having apair of adjacently positioned confronting static seals for sealablyengaging the intermittently fed thermoplastic material sheet, andwherein each of the two outer and central pressure chambers areseparably pressurable to selected first, second, and third pressures,respectively; pressurizing each of the two outer and central pressurechambers to the selected first second, and third pressures; andintermittently feeding the thermoplastic material sheet through each ofthe two outer and central pressure chambers by way of the interiorsealable pass-through slots.

These and other aspects of the present invention will become moreevident upon reference to the following detailed description andattached drawings. It is to be understood, however, that variouschanges, alterations, and substitutions may be made to the specificembodiments disclosed herein without departing from their essentialspirit and scope.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are intended to be illustrative and symbolicrepresentations of certain exemplary embodiments of the presentinvention. For purposes of clarity, like reference numerals have incertain instances been used to designate like features throughout theseveral views of the drawings.

FIG. 1 is schematic view of a semi-continuous method useful for makingsolid-state microcellular foamed rolls/sheets in accordance with theprior art.

FIG. 2 is an end view of an interleaved thermoplastic material rollbefore and during gas saturation in accordance with the prior art.

FIG. 3 is a side view of a pressure vessel having a rotatable shaft inaccordance with an embodiment of the present invention.

FIG. 4 is a side view of a pass-through, multi-chamber pressure vesselhaving a rotatable shaft in accordance with an embodiment of the presentinvention.

FIG. 5 is a sectional view of the interior door (closed and opened) ofthe pass through, multi-chamber pressure vessel of FIG. 4.

FIG. 6 is a side view of a thermoplastic roll being fed through apass-through, multi-chamber pressure vessel having a roller system (andbeing rewound into a gas impregnated roll) in accordance with anembodiment of the present invention.

FIG. 7 is a side view of a thermoplastic roll being fed through anelongated pressure vessel having an open-ended rectangular cavity andstatic seals (and being rewound into a gas impregnated roll) inaccordance with an embodiment of the present invention.

FIG. 8 is a sectional view of the rectangular cavity of FIG. 7.

FIG. 9 is a side view of a thermoplastic roll being fed through anelongated pressure vessel having an open-ended rectangular cavity andstatic seals (and being foamed by exposure to a heat source) inaccordance with an embodiment of the present invention.

FIG. 10 is a side view of a thermoplastic roll being fed throughpressure vessel having a roller system (and being rewound into a gasimpregnated roll) in accordance with an embodiment of the presentinvention.

FIG. 11 is a side view of a thermoplastic roll being fed throughpressure vessel having a roller system (and being foamed by exposure toa heat source) in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to various methods and pressure vesselsfor solid-state microcellular processing of thermoplastic rolls, sheets,and films. Thus, and with reference to FIG. 3, the present invention ina first embodiment is directed to a method for making a gas impregnatedinterleaved roll, which method comprises at least the following steps:providing a pressure vessel 1 having an internal pressure chamber and arotatable shaft 2 horizontally positioned within the pressure chamber;placing an interleaved roll (having a diameter ranging from about 10 toabout 16 inches) about the rotatable shaft 2 and within the pressurechamber, wherein the interleaved roll is made from a thermoplasticmaterial sheet (having a thickness of about 0.01 to about 0.1 inches)interleaved together with a gas-channeling material sheet (having athickness of about 0.01 to about 0.05 inches); pressurizing the pressurechamber to a selected pressure (with an effective amount of aplasticizing gas such as, for example, CO₂ or N₂); rotating therotatable shaft having the interleaved roll thereabouts (therebyrotating the interleaved roll) while under pressure for a selectedperiod of time; and depressurizing the internal chamber to yield the gasimpregnated interleaved roll. The pressure generally ranges from about0.345 MPa to about 9.65 MPa (or more preferably from about 3.5 MPa toabout 7.1 MPa), add the temperature is generally room temperature (butmay range from about −20° F. to about 150° F.). Depending on theselected thermoplastic material and roll diameter (which generallyranges from about 10 in about 16 inches), gas, pressure, andtemperature, the period of time generally ranges from about a few hoursto well over a hundred hours.

This method and related pressure vessel are intended for (but notlimited to) processing solid-state polymers with relatively slow gasdiffusivity, saturation time (T_(s)) greater than 10 hours, likepolyetherimide (PEI) and polyetherketoneketone (PEKK). The single-door,single-chamber pressure vessel 1 having a rotating spit or shaft 2 (FIG.3) in which one or more thermoplastic film rolls, interleaved with agas-channeling medium, are inserted through the shaft 2 inside thepressure chamber, treated with high pressure inert gas, and evacuated tobe foamed. This pressure vessel is specifically adapted for processinginterleaved thermoplastic rolls with slow gas diffusivity that mayexperience within-roll variation in gas concentration during absorption.The shaft turns the roll as it is being supplied with gas in order tocombat the effects of sagging and stress on the roll from the saturatinggas and loss of porosity in the interleave medium due to compressionfrom the roll's expanding volume, all of which can adversely impact gasabsorption and lead to undesirable variations in gas concentration ofthe thermoplastic roll.

In another embodiment and with reference to FIG. 4, the presentinvention is directed to a method for making a gas impregnatedinterleaved roll, which method comprises at least the following steps:providing a pressure vessel having an internal pass-throughmulti-chambered cavity, wherein the multi-chambered cavity includes atleast two outer pressure chambers sandwiching a central pressurechamber, and wherein a rotatable shaft is horizontally positioned withineach of the two outer and central pressure chambers of themulti-chambered cavity; placing an interleaved roll about the rotatableshaft and within one of the two outer pressure chambers, wherein theinterleaved roll is made from a thermoplastic material sheet (having athickness of about 0.01 to about 0.1 inches) interleaved together with agas-channeling material sheet (having a thickness of about 0.01 to about0.05 inches); pressurizing the one of the two outer pressure chamber toa first selected pressure; rotating the rotatable shaft having theinterleaved roll thereabouts while under the first selected pressure fora first selected period of time; conveying the interleaved roll to thecentral pressure chamber while under pressure; rotating the rotatableshaft having the interleaved roll thereabouts while under a secondselected pressure for a second selected period of time; anddepressurizing the other one of the two outer pressure chambers to yieldthe gas impregnated interleaved roll. The gas may be CO₂ or N₂, forexample. The pressures generally range from about 0.345 MPa to about9.65 MPa (or more preferably from about 3.5 MPa to about 7.1 MPa), andthe temperatures are generally room temperatures (but may range fromabout −20° F. to about 150° F.). Depending on the selected thermoplasticmaterial and roll diameter (which generally ranges from about 10 toabout 16 inches), gas, pressures, and temperatures, the period of timegenerally ranges from about a few hours to well over a hundred hours.

This method and related pressure vessel are intended for (but notlimited to) processing solid-state polymers with moderate gasdiffusivity (1 hr<T_(s)<10 hrs), like polyethylene terephthalate (PET),polystyrene (PS), polycarbonate (PC), and acrylonitrile butadienestyrene (ABS). As shown in FIG. 4, the pressure vessel includes anextended, pass-through multi-chambered cavity 1 segmented into threechambers 2, 3, 4 with a rotatable shaft 9 extending across all threechambers. The outer chambers 2, 4 sandwich the interior chamber 3. Inthis embodiment there are four sealed doors. The two solid exteriordoors 5 and 6 open into the outer chambers 2 and 4, respectively. Theinterior doors 7 and 8 allow access to chamber 3 as well as access tothe outer chambers 2 and 4, respectively. The interior doors 7, 8include dual panels that slide outward to open and inward to close (FIG.5). In one embodiment the interior doors are connected to hydrauliccylinders 12 used to pull open and to seal shut the doors (FIG. 4).

This pressure vessel is adapted specifically for processing interleavedthermoplastic rolls with moderate diffusivity that may experiencewithin-roll variation during absorption and between-roll variationduring desorption. Designed for large scale thermoplastic production,this pressure vessel allows interleaved thermoplastic rolls to heinserted, advanced, and removed from the pressure vessel on demand;performs uniform gas saturation via a rotatable shaft on multipleinterleaved thermoplastic rolls; and stores the rolls in a high pressureenvironment until they are ready for removal. In short, the design ofthe pressure vessel allows for better streamlining and control over theabsorption process and better regulation of desorption time through itsstorage and conveyance capabilities. For example, chamber 2 initiallyhouses the thermoplastic roll; chamber 3 is reserved for high pressuregas saturation and can store multiple interleaved thermoplastic rolls;and chamber 4 is reserved to remove the roll after saturation. Therotatable shaft 9 (with optional screw thread on which each roll may behung) is used to transport the roll from one chamber to the next and torotate the roll during saturation to allow for uniform gas absorption.One or more inlet valves 10 are stationed at each chamber to inject highpressure gas, and one or more outlet valves 11 in each chamber toevacuate the gas. Each chamber has its own separate controls (not shown)for its inlet and outlet valves. The doors allow entry into and exitfrom the chambers; the interior doors 7, 8 are designed to seal in theinterior chamber 3 in order to maintain chamber's 3 high pressureenvironment whenever any of the exterior doors 5, 6 from chambers 2, 4,respectively, is opened.

As an example, when the pressure vessel is operating, all three chambers2, 3, 4 are uniformly injected with high pressure gas at the same gasconcentration level and pressure. The exterior doors 5, 6 are closedshut and the interior doors 7, 8 are either opened or shut. When theinterleaved thermoplastic roll is first placed inside the pressurevessel from the outer chamber 2, the interior doors 7, 8 of the interiorchamber 3 are closed to seal in chamber's 3 pressurized environment. Gasfrom chamber 2 is evacuated and the exterior door 5 is opened to allowthe interleaved thermoplastic roll to be inserted through the rotatableshaft 9 inside chamber 2. High pressure gas is then re-injected intochamber 2 until it reaches the same gas pressure level as the interiorchamber 3, at which time the interior doors 7, 8 are opened, and therotating shaft 9 advances the thermoplastic roll into chamber 3, wherethe roll is saturated with high pressure gas until it reaches thedesired gas concentration. The thermoplastic roll sits in chamber 3until it is ready for removal, at which time the roll is advanced toouter chamber 4, the interior doors 7, 8 are sealed shut, gas fromchamber 4 is evacuated, and die exterior door 6 is opened to remove theroll. For the entire time, the interior chamber 3 is continuouslysaturated with high pressure gas to ensure a constant pressurizedenvironment so that the thermoplastic roll maintains its gasconcentration level.

In another embodiment of a multi-chamber pressure vessel (FIG. 6), thepressure vessel 1 includes an open-ended, three-chamber cavity 2, 3, 4with a system of rollers 9 that extends across all three chambers. Theends of the cavity are rectangular slots 5 with static seals 6 used toopen and seal shut the outer chambers 2, 4. The rectangular slot allowsfor a thermoplastic roll to be fed into the outer chamber 2 at one endof die pressure vessel, advanced to the interior chamber 3 to besaturated with high pressure gas, and stored until it is ready forremoval at the other end of the pressure vessel from the outer chamber 4through a rectangular slot. Like the embodiment of a multi-chamberpressure vessel, this embodiment includes an interior chamber with twodual-panel doors 7, 8 operated by hydraulic cylinders. The doorsmaintain the interior chamber's pressurized environment by sealing offthe chamber whenever the outer chambers are opened to insert or evacuatethe thermoplastic roll. There are also inlet 10 and outlet 11 valves foreach chamber.

In another embodiment and with reference to FIG. 7, the presentinvention is directed to an intermittent saturation system whereby asolid-state thermoplastic roll with fast diffusivity is intermittentlyfed through an open-ended rectangular cavity 2 with rectangular slot(FIG. 8) of a long pressure vessel 1. The pressure vessel is filled withsolid filler material that surrounds the cavity to eliminate thevolumetric space that would otherwise be saturated with high pressuregas. The rectangular cavity 2 stretches from one end of the pressurevessel to the other and is lined throughout with a porous gas-channelingmaterial to allow uniform absorption on all surfaces of thethermoplastic film. Static seals 3, located at opposite ends of thepressure vessel to seal shut the pressure vessel, integrally sandwichesthe thermoplastic film as high pressure gas is supplied to the pressurevessel to allow gas to diffuse evenly into the thermoplastic film untilit reaches the desired gas concentration level. When the film is readyto be removed for the next step, the pressure vessel is evacuated ofgas, the static seals are disengaged, and the film is advanced on theexit end to be re-rolled (FIG. 7). in another embodiment (FIG. 9), thesaturated film is passed through a flotation air oven 4 that injects hotair in through the air nozzles 5. The air nozzles 5, which arepositioned at the top and bottom of the oven, blow hot air as the filmpasses, causing the film to foam into evenly flat foamed thermoplasticsheets.

In yet another embodiment of an intermittent saturation system (FIG.10), a long pressure vessel 1 includes an open-ended cavity withrectangular slots 2 that extends across the length of the pressurevessel. A thermoplastic roll interleaved with a gas-channeling medium isfed into the cavity and passed through a system of rollers 3 like afestooned web. Again, static seals 4 located at opposite ends of thepressure vessel are used to clamp down the thermoplastic web and tosaturate the web with high pressure gas. The static seals aredisengaged, and the film is re-rolled on the exit end (FIG. 10) or, inanother embodiment, passed through a flotation air oven 5 (FIG. 11)where the hot air supplied by the air nozzles 6 causes the film to foaminto evenly flat foamed thermoplastic sheets.

The various embodiments disclosed herein provide alternative methods andapparatuses for addressing the problem of uneven gas concentration amongsolid-state thermoplastics with different gas diffusivity rates. A firstaspect relates to a pressure vessel having a rotatable shaft forpolymers with slow gas-diffusing rates. The shaft rotates theinterleaved thermoplastic roll as it is saturated with high pressure gasin order to prevent the roll from sagging and from compressing theinterleaved medium, factors which can lead to uneven gas absorption ofthe interleaved thermoplastic roll. A second aspect relates to amulti-chamber pressure vessel with a rotatable shaft or roller systemfor polymers with moderate gas-diffusing rates, which may experienceuneven gas concentration during the absorption and desorption phase. Themulti-chamber pressure vessel allows for greater flexibility and controlover the absorption and desorption process and is more responsive in itsability to insert, saturate, store, and evacuate a thermoplastic roll ondemand. A third aspect relates to a long pressure vessel with anopen-ended cavity and static seals for processing polymers withfast-diffusing, fast-desorblng rates. It provides an intermittentsaturation system whereby a thermoplastic sheet or web is inserted intothe rectangular cavity of the pressure vessel or through a system ofrollers, fastened down by static seals, and uniformly exposed to highpressure gas.

While the present invention has been described in the context of theembodiments illustrated and described herein, the invention may beembodied in other specific ways or in other specific forms withoutdeparting from its spirit or essential characteristics. Therefore, thedescribed embodiments are to be considered in all respects asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription, and all changes that come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

1.-13. (canceled)
 14. A method for gas impregnation of an intermittentlyfed thermoplastic material sheet, comprising: providing a pressurevessel having at least two outer pressure chambers sandwiching a centralpressure chamber, wherein the central pressure chamber is separated fromeach of the two outer pressure chambers by respective interior sealablepass-through slots, wherein each of the interior sealable pass-throughslots is defined by a rectangular opening having a pair of adjacentlypositioned confronting static seals for sealably engaging theintermittently fed thermoplastic material sheet, and wherein each of thetwo outer and central pressure chambers are separably pressurable toselected first, second, and third pressures, respectively; pressurizingeach of the two outer and central pressure chambers to the selectedfirst, second, and third pressures; and intermittently feeding thethermoplastic material sheet through each of the two outer and centralpressure chambers by way of the interior sealable pass-through slots.15. The method of claim 13 wherein the selected first, second, and thirdpressures each range from about 3.5 MPa to about 7.1 MPa.
 16. The methodof claim 15 wherein the selected first, second, and third pressures areeach about the same.